If it has a white line it IS marked. The white line indicates the negative terminal. Sometimes there is a minus sign within the white line. If the capacitor has a light coloured case then it's usually a black line that indicates the negative terminal. On vertically mounting caps ( both leads on the same end) the side the line is on is the negative. On horizomtal mounting caps ( leads are at opposite ends) there should be an arrow within the line, pointing to the negative or the line is nearer the negative end of the case. With the latter, you can also see that the negative side is the metal casing.
Its a long time since I saw the positive terminal marked on small electrolytics.
Its different with tantalum capacitors, with these they mark the positive side with a '+' or a stripe. But 100micro would be quite a large tantalum so we can safely assume you got an electrolytic capacitor.
Make sure you do not exceed the peak voltage rating, which should be marked on the case.
Klaus